Waveguide directional couplers are widely used in applications such as the combining of power from a plurality of low power signal sources to generate higher power signals, for signal sampling, for comparing the power applied to and reflected from an antenna, in beam forming networks in multi-horn antenna arrays, in non-contact phase shifting switching schemes, and the like.
In general, a waveguide branch directional coupler includes two generally parallel through waveguides which are connected together by two or more branch waveguides. The relative amount of power coupled from one of the through waveguides to the other is determined by the amount of power flowing in the branch waveguides, which, in turn, depends upon the relative dimensions of the branch waveguide and the through waveguide to which the signal is applied. The couplers are often manufactured by machining slots into a solid block of metal to form a conductive housing. A separate cover closes the housing.
Various waveguide directional couplers are set forth in U.S. Pat. Nos. 4,679,011 (Praba et al ), issued July 7, 1987, U.S. Pat. No. 4,127,831 (Riblet), issued Nov. 18, 1978, and U.S. Pat. No. 4,792,770 (Parekh et al.), issued Dec. 20, 1988.
The Praba et al. patent discloses a waveguide branch directional coupler which includes a conductive housing which defines first and second mutually parallel waveguides and a chamber which extends therebetween. The width of the chamber is equal to the width of the waveguides. The length of the chamber in a direction parallel to the axes of the waveguides is fixed for all members of a family of coupling values. The coupler includes one or more rectangular conductive blocks dimensioned to extend between the waveguides. The conductive blocks are so dimensioned that when fastened in place within the chamber they define two or more branch waveguides extending between the parallel waveguides.
The Riblet patent discloses a symmetrical two branch coupler comprised of four sections of signal transmission line interconnected so as to form at the junction therebetween four ports of the coupler with oppositely disposed lines having like characteristic admittances. The two branch coupler includes at least two 2-port matching networks connected at two of the four ports of the coupler with each matching network connected at its associated port and independent of connection to the other ports.
The Parekh et al. patent discloses a directional coupler which includes a first waveguide and a longitudinal septum dividing the first waveguide into plural longitudinal channels at least in a coupling region. Additional waveguides are coupled by directional coupling apertures to the longitudinal channels of the first waveguide.
Whatever the merits of the above-cited patented devices for their intended purposes, they are not viewed as particularly pertinent to the context and objectives of the present invention.
In the context of a conventional weather radar system, it has been found that providing conventional multi-hole sidewall couplers having a coupling factor of -20 dB results in an insufficient signal at the weather channel. The multi hole sidewall coupler typically diverts 1% of the incoming power (r.f. energy) to the weather channel, whereby the diverted incoming power is so small that nothing can be seen at the weather channel.
FIG. 1 attached hereto depicts a typical radar system 1 which includes antenna 2, circulator 3, multi-hole sidewall coupler 4, weather channel 5, and target channel 6. Incoming power from antenna 2 passes through circulator 3 and is loaded in coupler 4, wherein the approximately 1% of the incoming power is diverted to weather channel 5, an amount totally insufficient for viewing purposes.
In response to the above-stated problem of insufficient power in the weather channel. It might be thought that the most cost-effective solution would be simply to couple more power into the weather channel. The difficulty in a multi-hole sidewall coupler is that such a solution involves increasing the length of the coupler substantially.
Accordingly, it is a primary object of the present invention to realize an efficient directional coupler for use in weather radar systems by providing a compact device capable of producing sufficiently high power levels in a subsidiary channel, while insuring high directivity.
Briefly, the primary feature of the present invention derives from the recognition that a known, short slot, 3 dB hybrid waveguide can be successfully exploited. By appropriately decoupling the slot using judiciously placed first and second posts in the junction defined by the slot, the basic objectives sought can be achieved. Further, the depth of an "inverted" button, as it is sometimes called which is in the form of a capped-off hole or holes, is used to insure that the requisite directivity is achieved. It will be appreciated by those skilled in the art as the description proceeds that there is a balance required between the aforenoted dimensions for any given coupling value, that is, the dimensions involving spacing between posts, and the depth of the capped-off holes.
The directional coupler according to the present invention also provides the following advantages over conventional multi- hole couplers: it can be used with varying frequencies to achieve coupling, from -3 dB to -15 dB, to a subsidiary channel; it allows for controlling directivity over varying frequencies; it avoids having to manufacture or replace couplers for varying frequencies; and it can use the full power capability of coupler waveguides, while still remaining compact.
A substantial advantage of this type of compact waveguide coupling device is in the formation of antenna beam forming networks (BFN) for both earth station and satellite applications. The size reduction allows for a much more compact BFN giving maneuverability advantages in earthborne applications. The size and, more importantly, the weight differs immense advantages in satellite BFNs where any weight reduction extends the useful life of a satellite.
Additional advantages of the present invention shall become apparent as described below.